Steam power plants contribute decisively to the stabilization of voltage and frequency both in interlinked networks and in island networks. In order to meet these stabilization requirements, the control strategies of steam power plants must fulfill the highest possible demands. The control strategies, in this context, are especially important in the event of network accidents and rapid load changes.
If, for example, the rotation of the generator deviates sharply from the nominal value and the machine runs the risk of slips or the shafting of the generator and turbine is put at risk by rotational overspeed, the entire steam power plant has to be decoupled in a directed manner from the associated network and run down to its own requirements so that it is available again as quickly as possible for the network configuration. After such load shedding, the power at the terminals of the generator is reduced in a short time to low values. So that the shafting is not accelerated excessively due to such a diminution in the actual power of the generator, valves of the associated turbine have to be shut quickly. After load shedding, the electrical power of the terminals of the generator generally remains at a low value for a lengthy period of time.
By contrast, the accident referred to below as a short circuit interruption is a usually 3-pole network short circuit in the vicinity of the power plant which lasts for only a few 100 ms. In the event of such a network accident, the power at the terminals of the generator is briefly equal to zero on account of the voltage collapse mentioned. Insofar as the short circuit can be extinguished within a fault clear-up time of at least 150 ms, the generator will continue to feed active power and reactive power into the network in order to stabilize frequency and voltage. Hence, if the short circuit is present for 150 ms or a shorter time, the shafting should not slip nor should the associated turbine be run down. In many steam power plants, the possible fault clear-up time is even markedly shorter.
The control of a steam power plant must react to both accidents, the problem being that the power shedding and the short circuit interruption cannot be distinguished at the commencement of each of these, since, in both cases, the power at the terminals of the generator falls. Furthermore, there is the problem that, although, in the case of short circuit interruption, the electrical power returns after the fault clear-up, whereupon the turbine would have to continue to be operated, nevertheless, as time goes on, the electrical power often swings through its zero passage, and therefore, if predefined power limit values are undershot, movement controllers detect an accident once again. With each accident detection, particularly in known steam power plants, the power of the associated turbine is reduced in that associated valves are shut quickly. On account of said swing of the generator active power about the zero point after a short circuit interruption, such rapid valve motion of the steam turbine may experience a frequent successive response. As a result, the turbine power and therefore the feed of active power into the network are greatly reduced for a disproportionately long time of several seconds.
If this problem arises in several steam power plants, it leads to unacceptable load flow and frequency problems. In the event of faults of this kind, the steam power plants must ensure the frequency and voltage stability of the network within a time range of a few 100 ms.